Serbia’s pro-western prime minister, Aleksandar Vučić, won a resounding endorsement in Sunday’s general election for his policy of pursuing European Union membership, securing four more years in power with a parliamentary majority.

But he will have to contend with a resurgent ultra-nationalist opposition that rejects integration with the EU and demands closer ties with Russia.

Vučić went to the polls two years early, saying he wanted a clear mandate from Serbia’s 6.7 million voters for reforms to keep EU membership talks launched in December on track for completion by 2019.

Even though Vučić presided over a period of austerity, partly forced on him by the terms of a 1.2bn euro ($1.35bn) loan agreement with the International Monetary Fund, voters again strongly backed the 46-year-old, himself a former hardline nationalist.

His conservative Progressive party is set to win just under 50% of the vote, up from 48% two years ago, a projection by pollsters Cesid, the Centre for Free Elections and Democracy, said.

“This is an historic result, getting more votes in absolute numbers and in percentages than two years ago when we started difficult reforms,” said Vučić.

“Today’s result strongly supports our democracy, diplomatic efforts and European integration,” he said.

Vučić must now decide whether he will rule alone or seek to broaden his support further by continuing to govern in coalition with the Socialists, who came second with about 11.6% of the vote, or another party.

The election marked a resurgence by the ultra-nationalist Radical party of Vojislav Šešelj, who was acquitted by the UN tribunal in The Hague last month of war crimes during the 1990s breakup of Yugoslavia.

The Radicals were set to win about 7.8% of the vote, turning them into the third-biggest party in parliament after a four-year absence from the assembly.

They could turn into a thorn in Vučić’s side, resisting his pro-EU policies and calling instead for an alliance with Russia.

Šešelj voiced disappointment with the result but said “in future debates we will show we are superior to our opponents”.

The pro-EU Democratic party, which won about 6% of the vote, complained of scattered irregularities that favoured the Progressive party, saying some voters had been given ballots that were already filled in.

Exactly how many seats in the 250-member parliament the Progressives end up with depends on how many other parties exceed the 5% threshold needed to get into the assembly.

Three parties are hovering around 5%, according to Cesid. If they all get into parliament, it would reduce the Progressives’ majority.

But analysts said the Progressives were still likely to get an absolute majority of between 137 and 156 seats, compared with 158 in the old parliament.

The EU commissioner in charge of relations with would-be member states, Johannes Hahn, said on Twitter he was confident Vučić “will use citizens’ strong support in a responsible way and that it [the election] will strengthen Serbia’s EU perspective.”

Milan Jovanović, a political science lecturer at Belgrade University, said the Radicals would not be able to significantly influence the government’s behaviour. “Quite to the contrary, they could make the government even more determined,” he told Reuters.